Some questions about the MoorDyn module

Dear Jason:

I am using the FAST v8 to model the offshore wind turbine and the semi-submersible type of floating structure (OC4 DeepCwind in Test 25) is used. In fact, I aim to investigate the dynamics of wind turbine in different water depth such as 50m, 100m, 200m, 300m etc (We assume it is feasible to estabilish the floating wind farm in deeper water). To the best of my knowledge, the mooring system need be changed with the variance of water depth. More specifically, the MoorDyn input file should be changed with the water depth. The questions are presented as follows.

  1. According to the MoorDyn user’s guide, it seems that I need to change the CONNECTION PROPERTIES flag when the water depth varies. So I only to modify the values of Fixed Type lines in CONNECTION PROPERTIES flag. An example of my modification is included as follows. I am not sure if I need to modify other flags. Could you help me?

---------------------- CONNECTION PROPERTIES --------------------------------
6 NConnects - number of connections including anchors and fairleads
Node Type X Y Z M V FX FY FZ CdA CA
(-) (-) (m) (m) (m) (kg) (m^3) (kN) (kN) (kN) (m^2) (-)
1 Fixed 128.22 221.97 -61.23 0 0 0 0 0 0 0
2 Fixed -256.44 0.0 -61.23 0 0 0 0 0 0 0
3 Fixed 128.22 -221.97 -61.23 0 0 0 0 0 0 0
4 Vessel 20.434 35.393 -14.0 0 0 0 0 0 0 0
5 Vessel -40.868 0.0 -14.0 0 0 0 0 0 0 0
6 Vessel 20.434 -35.393 -14.0 0 0 0 0 0 0 0

  1. But actually I don’t know how to calculate the values in CONNECTION PROPERTIES flag according to the water depth, and I guess the Vessel Type lines corresponding to the shapes of OC4 DeepCwind model are need not changed. If I only scale up/down the orginal values in Test 25, some errors appear when I run some simulation cases.

Creating mooring system. 3 fairleads, 3 anchors, 0 connects.
Catenary solver failed for one or more lines. Using linear node spacing.
Catenary solver failed for one or more lines. Using linear node spacing.
Catenary solver failed for one or more lines. Using linear node spacing.
Finalizing ICs using dynamic relaxation.

t=2 FairTen 1: 39649, 0, 0
t=3.9999 FairTen 1: 46631, 39649, 0
t=5.9997 FairTen 1: 49323, 46631, 39649
t=7.9996 FairTen 1: 52435, 49323, 46631

Fairlead tensions did not converge within TMaxIC=60 seconds.

And I fail to solve this problem and seek your help.

  1. In fact, the modeling of mooring system is not the focus of my research, but the dynamics of mooring system should be included because it will influence the dynamics of wind turbines. So could you tell me how to simplify the mooring system for crude calculation of mooring dynamics?

Thanks for your helping me all the time.

Best regards,

Dear Yichau.Liu,

I’m not sure what you mean when you say that you “only scale up/down the original values in Test25.” I would think that at a minimum you’d need to change the X,Y,Z positions of the Fixed (anchor) nodes) in the CONNECTION PROPERTIES section and the unstretched length (UnstrLen) in the LINE PROPERTIES section. For very large changes in water depth, a redesign of the mooring system would likely be required.

Best regards,